A typical classified document. Page 13 of a U.S. National Security Agency report [1] on the USS Liberty incident, partially declassified and released to the public in July 2003. The original overall classification of the page, "top secret", and the Special Intelligence code word "umbra," are shown at top and bottom. The classification of individual paragraphs and reference titles is shown in parentheses - there are six different levels on this page alone. Notations with leader lines at top and bottom cite statutory authority for not declassifying certain sections.

Classified information is sensitive information to which access is restricted by law or regulation to particular classes of people. A formal security clearance is required to handle classified documents or access classified data. The clearance process requires a satisfactory background investigation. There are typically several levels of sensitivity, with differing clearance requirements. This sort of hierarchical system of sharing information among a group of people is almost used by every nationalgovernment. The operation of assigning the level of sensitivity to data is called data classification.

The purpose of classification is to protect information from being used to damage or cause danger to national security. Classification formalizes what constitutes a "state secret" and deals with different levels of protection based on the expected damage the information might cause if it reached the wrong hands.

Certain non-government organizations and corporations also have classified information, normally rather referred to as trade secrets.

Although the classification systems vary from country to country, most have levels corresponding to the following British definitions (from the highest level to lowest):

Top Secret (TS)

The highest level of classification of material on a national level. Such material would cause "exceptionally grave damage" to national security if publicly available.

Secret

Such material would cause "serious damage" to national security if publicly available.

Confidential

Such material would cause "damage" or be "prejudicial" to national security if publicly available.

Restricted

Such material would cause "undesirable effects" if publicly available. Some countries do not have such a classification.

Unclassified

Technically not a classification level, but is used for government documents that do not have a classification listed above. Such documents can sometimes be viewed by those without security clearance.

Depending on the level of classification there are different rules controlling the level of clearance needed to view such information, and how it must be stored, transmitted, and destroyed. Additionally, access is restricted on a "need to know" basis. Simply possessing a clearance does not automatically authorize the individual to view all material classified at that level or below that level. The individual must present a legitimate "need to know" in addition to the proper level of clearance.